The invention relates to the urea-dewaxing of mineral oils containing very small amounts of n-paraffins. More particularly, the invention relates to the production of refrigerator oils from such low paraffin content mineral oils.
It is known to dewax hydrocarbon mixtures or mineral oil distillates by reacting them with urea to form solid inclusion compounds or adducts of the n-paraffins and separating these adducts from the dewaxed mineral oil. This process is called urea dewaxing and has been used in different embodiments for the large-scale refining of mineral oil. Usually in this process, the mineral oil distillate charge is diluted with an organic solvent or mixture of solvents in order to reduce the viscosity of the reaction mixture, to improve its pumping and mixing properties, and to increase the degree of separation of the adduct and the dewaxing selectivity. In most cases, the same solvent, or solvent mixture, is used for washing the separated adducts and, if desired, for extracting the n-paraffins from the adduct. The solvent, or solvent mixture, should not form adducts with urea under the reaction conditions being employed. Conventional solvents include lower aliphatic and aromatic hydrocarbons, chlorinated hydrocarbons such as, dichloromethane, and the like.
Usually, the urea is dissolved in water or methanol, the solution having been saturated at a temperature above the predetermined reaction or adduct-forming temperature. The amount of urea required for adduct formation is from 3.5 to 4 parts by weight per part by weight of n-paraffin to be separated from the mineral oil. It has also been proposed to use urea in crystalline form, preferably finely divided. In this case the formation of adduct progresses at a very slow rate, therefore usually minor amounts of so-called activators such as water, methanol, ketones, and other urea-dissolving agent are added.
It is also known to initiate or accelerate the adduct-forming process by introducing a so-called "seed adduct". Another known method of enhancing the formation of adduct is by vigorously mixing the reaction mixture. However, vigorous mixing of such mixtures, especially aqueous ones, frequently tends to form emulsions which can be broken only with great difficulty.
With the known urea dewaxing processes it may be difficult to separate the adducts from the solution of the dewaxed mineral oil since the adduct often is in the form of a dust-like powder or a slimy-sticky mass which does not lend itself to filtration. Powder-like adducts may be separated by centrifuging inasmuch as there is a sufficient difference in gravity between them and the solution. There have also been attempts to remove the adduct, especially slimy adduct, from the solution by allowing it to settle. However, this remedial measure would require large volume settlers and would be so time-consuming that it could hardly be useful in a continuous commercial operation.
The above described difficulties of the prior dewaxing processes become more acute when the mineral oil distillate charge comprises only minor amounts or just traces of n-paraffins.
It is the object of this invention to improve the urea dewaxing process so that very small amounts of n-paraffins contained in mineral oil distillate and similar hydrocarbon mixtures may be removed therefrom in a simple relatively rapid manner, and that the removal is practically complete and is preferably performed in a continuous operation. Another object is to produce a lubricating oil for refrigerating machines from a naphthenic mineral oil distillate.